Hematologic disorders such as the thalassemias and hemoglobinopathies, resulting from absent/reduced or abnormal production of one or more of the globin-molecule subunits, respectively, together constitute the most prevalent group of human monogenic diseases. Strategies which aim to replace the absent or defective globin gene have long been envisioned as potentially curative, and gene transfer strategies targeting hematopoietic stem cells have been central to this goal. Certainly, allogeneic bone marrow transplantation, a form of hematopoietic stem cell based gene transfer accomplished by replacement of the entire diseased organ with that from a donor with a normal genotype, has proven curative, yet procedural toxicities limit application. In order to expand application, we have explored nonmyeloablative transplant regimens which are designed to allow engraftment of allogeneic hematopoietic stem cells without the toxicity of conventional marrow ablative conditioning. Using mobilized peripheral blood stem cells as the source, we demonstrated reliable engraftment in the absence of marrow ablation in patients with metastatic cancer and extended these observations to patients ineligible for conventional myeloablative transplantation due to comorbidities. While clearly establishing the ability to achieve hematopoietic engraftment in humans without marrow ablation, procedural toxicity, mainly in the form of graft-versus-host disease, remained too high for application to nonmalignant disorders. We therefore returned to animal models and have recently developed a low intensity conditioning regimen designed to promote tolerance to the allograft. Based upon a unique mechanism for tolerance induction, we compared the use of immunosuppression with rapamycin to that with conventional immunosuppression with cyclosporine after low dose irradiation in a murine model of mobilized peripheral blood allograft rejection. Only mice treated with rapamycin demonstrated long-term hematopoietic chimerism, and the levels achieved exceeded 75% at greater than 4 months of follow up. In anticipation of moving these observations toward clinical application for adults with sickle cell anemia, we established the safety and feasibility of peripheral blood stem cell mobilization in individuals with sickle cell trait, as these heterozygotes represent approximately half of the sibling donor pool. We initiated a clinical trial for adults with sickle cell anemia and thalassemia and recently reported our results in the first 10 patients. Since that report, accrual has reached over 30, and results are similar with an 86% disease free survival. The protocol has been amended to accrue up to 50, with a number of secondary endpoints such as neurocognitive functioning measured before and yearly with their sibling donor as the control, pain, quality of life, kidney function, lung function, heart function. Additionally, we have begun a planned immunosuppression taper in individuals with >50% CD3+ T cell chimerism, and the majority of subjects are now off immunosuppression with stable mixed chimerism. There has been no acute or chronic graft versus host disease, and the mixed hematopoietic chimerism observed in the absence of long term immunosuppression demonstrates operational tolerance. These results have also now been reported. We have now determined the level of chimerism sufficient to correct the phenotype, modeled it mathematically, and the results are published in Blood. We are almost at our accrual ceiling of 50 successfully transplanted patients, and this will allow us to complete our analysis of secondary outcomes including extensive neuropsychological testing in patients during follow up compared to their sibling donor. The first, cross-sectional analysis of the data has been submitted for publication. Given the limited number of patients with an available HLA-matched sibling donor, we have also moved on to test this approach in the haplo-idendtical setting in a protocol testing escalating doses of post-graft cyclophosphamide. Our first protocol testing this was just published this year and a new protocol is now active. Accrual to the new protocol is now ongoing, and the protocols now being run by a new tenure-track investigator in our group, Dr. Courtney Fitzhugh, and will be reported upon by her.